gas boiler hot water heating water pressure too low?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by lmei007, Oct 17, 2013.

  1. lmei007

    lmei007 Member

    Messages:
    178
    Location:
    Maryland
    I marked the water pressure on the gauge in my gas hot water heating system last year. It was about 20psi. I just started to use it this year. I noticed the pressure on the gauge is much lower than last year. It's a little higher than 10psi after the system is heated.

    The system is connected directly with the house water system with an opened ball valve.

    Does that mean the house water pressure is that low?

    Somebody said online that the heating system pressure should be in the range of 12 -- 22 psi. Is it right? Should I bring up the pressure if I can (I do a have pressure regulator installed at the water entrance of the house)?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,153
    Location:
    New England
    If your water pressure in the whole house was 10psi, you'd know it! Water would trickle out of the faucets and your shower would be a dribble.

    Every boiler system has a minimum pressure it requires, and most (if not all) have a safety cutout switch to stop it from firing if the pressure is too low. The manual should say what you need...10psi may be a little low. Just like in a car's radiator, you need some pressure to prevent the water from exploding into steam.

    I seriously doubt your boiler is directly connected to the house's water supply without something in between. There has to be a valve of some sort, and often, there's an autofill valve (but it sounds like you don't have one of those, or if you do, it is either defective or misadjusted). There should be a backflow preventer between the boiler and the potable water supply, otherwise, you could get stagnant, metal saturated, nasty water back into your drinking water.

    You may find, if the boiler will fire, that the pressure increases when it is running because the water expands. I'd try that first before I added any water to the system. The expansion tank tends to limit that rise, but there may be some.

    Last thing I can think of, the pressure gauges aren't always that accurate and sometimes stick a little. Try tapping it and see if the pressure changes.

    Having a screw-on pressure gauge so you can check the house pressure isn't a bad idea to have around...typically in the $10 range. Get one with a second, tattle-tale hand that will show peak pressure over time. You can use it on the boiler, too, attach to one of the drain ports, and open it. That would help confirm whether the gauge on the boiler is working right.
  3. Dana

    Dana In the trades

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    Having the system connected directly to the house water system with an opened ball valve would be a code violation unless there were check-valve to prevent back flow too. Most boilers are installed with pressure relive valves that open at nominally 30 psi, which is lower than most potable water systems. If it were connected to the potable distribution system without a pressure regulating auto-fill valve at the boiler you'd probably notice the knee-deep water in the basement, eh? ;-)

    The heating system should never be set to the house pressure. The system is usually filled either with a pressure-settable/adjustable auto-fill valve or manually using a gate or ball valve, setting the correct system pressure when the system is cold. Running it at 8 psi or lower when cold is too low, since it is low enough for oxygen to get into the system, causing system components to rust. At 12 psi (cold system pressure) you're good for most 1 & 2 story houses.

    I suspect you have an auto-fill valve between your open ball valve and the rest of the system that has either jammed and stopped working or needs re-adjustment, or there is another valve that's closed. Normally the cold system pressure will not drop until/unless there is a leak on the system, or the expansion tank has lost some charge. Auto-fill valves are something of a "solution-problem"- they keep the pressure from dropping even if there IS a leak, and the leak goes undiscovered until it's pretty big and making a mess somewhere. On my own system I prefer to fill the system to the correct pressure then turn off the ball valve. That way I'll know of the system is leaking somewhere if it starts loses pressure over time, and I can go looking for it. If the pressure drops super low there will be plenty of sizzle & bang noises coming out of the boiler when it's running before it's a safety issue.
    Last edited: Oct 17, 2013
  4. hj

    hj Moderator & Master Plumber Staff Member

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    The pressure in a boiler is "height sensitive". In other words you need at least enough pressure to raise the water to the level of the highest radiation in the building. A single floor structure COULD operate with 10psi since that would raise the water approximately 20 feet with a little extra as a cushion.
  5. lmei007

    lmei007 Member

    Messages:
    178
    Location:
    Maryland
    I am wrong, sorry. The boiler is NOT directly connected to the water supply. There are at least two devices in between. Pictures will be a better way to show the status.

    The blue pex is the pipe connected to the house water supply.

    I tested the house water pressure last night. The reading is about 62psi. My gas boiler is a 3 yrs old Burnham Alpine high efficiency boiler. I am just wondering why the heating loop pressure reduced from 22psi last year to about 12 psi this year, and how to bring it back? The 12 psi is read after the system is fully heated. My house is a one floor house.

    Attached Files:

    Last edited by a moderator: Oct 18, 2013
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,153
    Location:
    New England
    Well, since we're now talking apples to apples, if the pressure dropped, there's a (very) small leak somewhere. It could easily be so small that you wouldn't notice. Check all connections you can and look for mineral buildup or tracks. A drop or two a day won't wet or cause rot, but over time can lower the pressure. As things cool off, they shrink, and it could have lost some water when things were off (do you have an indirect WH? If you do, the thing has been cycling all year and this may not apply). A joint that is tight when it is hot, could leak a little when cold. The circulator may leak a little around the seal of the motor shaft. A seal may weep anywhere, as could the over pressure safety valve.
  7. lmei007

    lmei007 Member

    Messages:
    178
    Location:
    Maryland
    If it is leaking bring the pressure down, that is not a good news.
    I think the system should keep the pressure if the leak is very small. One of the device between the water supply and the heating loop does the job to regulate the pressure of the heating loop. Only when the leak is big enough, the loop cannot be recharge quick enough then the pressure will show lower. Am I right?
  8. Dana

    Dana In the trades

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    The bronze valve with the little flip-lever on top is the auto fill valve. The other thingy is probably the check-valve/backflow preventer, and they were probably sold together as a unit:

    [​IMG]


    http://s3.pexsupply.com/product_files/0386460-Submittal.pdf



    If you fiddle the little lever on top it should cam-open the valve, over-riding the preset. The screw on the top is a pressure adjustment, but make sure it's not just sticking first. If the auto fill valve is stuck on some grit or corrosion, manually opening may be enough to open it up and get it operating again, but if nothing else you'd be able to use it to manually fill the system. If it's still sticky, won't adjust up/down, you may want to take it out and open it up for closer inspection/cleaning- it's a bit young to have jammed from corrosion, so cleaning it up might do the trick.

    Unless yours is a 3+ story house, the system SHOULD be set to ~12 psi when the system is cold, maybe 15 psi with some low-mass boilers. When the system is hot, or even warm, the pressure will rise slightly, but if the expansion tank was sized correctly for the volume of water in your system, it shouldn't rise more than ~8 psi when the system is at it's hottest (the coldest night of the year when the outdoor reset curve is calling for 180F or whatever.) If the system is set to 22 psi whin cold, there's a good chance it'll blow-over the 30psi point, opening up the pressure relieve valve on the boiler, spitting/dumping scalding water in the process, then when the system cools it'll be at a lower pressure- the only system leak being the pressure relief valve doing what it was designed to do.


    The expansion tank needs to be pre-charged with air to that pressure too (which should be done when the system is de-pressurized.) Expansion tanks occasionally fail early in life, and installers occasionally neglect to pre-charge them correctly.
  9. WorthFlorida

    WorthFlorida USCG escorting cruise ship leaving Port Everglades

    Messages:
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    Location:
    Lake Worth, Florida
    Check out this link. It talks about pressure, it reads 12 PSI and it has a tutorial on how to recharge the expansion tank.

    http://homerepair.about.com/od/heatingcoolingrepair/ss/trblsht_boiler_5.htm

    Everything I read on auto fill valves is they are also a pressure regulator, that is it will allow water to flow when the pressure get blow the set point. Typically 12-15 PSI. What may have happened is when the unit was installed the water might have been turn on and allowed to fill the system, radiators open to bleed off any trapped air, then the expansion tank might have been filled with air pressure. Hence higher than normal pressure. But where this pressure go? Maybe the temp/pressure valve opened and you never noticed it after it dried or was any of the radiator's bleed to be sure there was no air in the system. A task my father and I did at the beginning of every winter. In fact just last week visiting my dad in NY, now 86 years old, asked me to bleed one radiator just to be sure where he had some doubt that the rad was not getting hot enough.
    Last edited: Oct 21, 2013
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